During the next period, we project four major studies as follows: 1. The development and exploitation of a system selective for mutants deficient in the excision-resynthesis aspects of repair or correction of meiotically generated hybrid DNA. 2. We will develop a totally analyzable hybrid heterozygous at three maximally separated sites within the arg4 locus. This will allow for assessments of the effects of heterozygosities on the correction at a given site. 3. Unselected tetrads in large numbers will be analyzed from a system that will permit precise localization of crossovers associated with gene conversion. The data should permit refinement of present views concerning polarity in recombination. 4. All rad mutants (radiation sensitive) known to be sporulation competent in the homozygous state will be tested for their effect on meiotic correction of hybrid DNA at several different test loci. The rad mutants are variously defective in repair and we seek to determine if any of them are deficient in recombinational repair.